The invention relates to starch-based polymer dispersion, which contains a co-polymer of starch with monomers. The invention also relates to a process for the preparation of said dispersion and its use in paper manufacture, particularly as a surface sizing additive for paper and as a wet- and dry-strengthener for paper which is added to the wet end of a paper machine. The term paper as used in this invention refers to both paper and paperboard, which are prepared by using either a wood-containing or a chemically processed fibre or a recycled fibre or a mixture thereof. These papers include fine papers, LWC papers, SC papers and newsprint papers, packing boards and folding boxboards.
U.S. Pat. No. 4,301,017 (Standard Brands Inc.) discloses an aqueous polymeric dispersion, which is produced by co-polymerization of at least one vinyl monomer in an aqueous solution of a thinned/degraded starch derivative. The starch derivatives used were diethylaminoethyl, acetyl, cyanoethyl or carbamoethyl derivatives. Such derivatives are just slightly ionic at neutral pH values, and in papermaking processes, best functioning is achieved only at low pH values (pH less than 6) in which they are partly ionized. Nowadays, most papermaking processes are neutral, and, consequently, polymers should also act at pH values in the range from 6 to 9, preferably from 7 to 8.5. The degree of substitution of the starch used in the above mentioned patent and having the above mentioned substituents should be at least 0.05. Before polymerization, starch is degraded/thinned enzymatically to a preferable intrinsic viscosity value ranging from 0.12 to 0.28 dl/g. Ce(IV) salts have been used as a polymerization catalyst (initiator).
Patent DE 37 02 712 (U.S. Pat. No. 4,835,212; BASF AG) describes the preparation of polymer dispersions from starches having an intrinsic viscosity of from 0.04 to 0.12 dl/g. Also the starches used in this patent are enzymatically highly degraded before the polymerization. In the examples, slightly cationic commercial starches have been used which have a maximum degree of substitution (DS) of 0.07. As monomers, in patent DE 3,702,712 it was used from 40 to 140 parts by weight, based on the total polymer, of a monomer mixture containing from 20 to 65% by weight of acrytonitrile or methacrylonitrile, from 80 to 35% by weight of acrylic acid esters and from 0 to 10% by weight of other co-polymerizable monomers. Peroxides were used as catalysts.
It is an object of this invention to provide dispersions which have a novel composition and an improved reactivity in comparison with the prior known dispersions, and a good retention and adherence to paper fibres. The dispersions according to the invention can be used as an additive in paper surface sizing and as a wet- and dry-strengthener for paper which is added to the wet end of a paper machine. The invention also relates to a process for the preparation of said dispersions.
In surface sizing, different kinds of polymers are generally added to the surface-sized starch in order to improve printability. These polymers can be copolymers of styrene with maleic acid, various acrylates etc. In these cases, the effect of the expensive polymers added will not be totally utilized, while the polymers are not completely inter-miscible with starch (thermodynamic solubility coefficients differ too much from each other), and thus they do not form a completely inter-miscible polymeric network (interpenetrating network) while drying.
It is one object of the present invention to produce polymer particles which have a surface structure similar to that of the used surface sizing starch, and, consequently, these two achieve a completely interpenetrating network. This is achieved by using as a polymer intermediate and a starting batch the same starch as or a starch chemically similar to the starch in the surface sizing agent itself. Then the particles formed during the polymerization remain in the starch film and are not separated during drying. If the starch is enzymatically degraded before the polymerization, other means are needed than only the starch itself for stabilizing the polymer, for example an emulsifier or water soluble monomers. In that case, the surface structure of the polymer is no longer similar to that of the surface sizing starch.
A copolymer suitable to be added into the pulp on a paper machine can be prepared by customizing for each paper machine a starch derivative having a cationic value and other properties suitable for the wet end system of the machine. Also herein, the guiding principle is that the surface structure of the polymer particles is as similar as possible as the molecular structure (molecular weight, cationic value, branching etc.) of the starch or the starch-based strengthener used at the wet end. Then the polymers are not separated from the starch matrix while drying but form a completely interpenetrating network.
Unlike in the two processes mentioned above, in the present invention it is possible to produce copolymers by using non-degraded or only slightly oxidized (e.g. peroxide- or hypochlorite-oxidized) starch. For example, the starch can be potato starch, corn starch, barley starch, wheat starch or tapioca starch, of which the potato starch and corn starch are preferable. The starch is not enzymatically degraded before the polymerization, and thus it has an intrinsic viscosity of at least 1.0 dl/g, preferably from 1.5 to 15 dl/g, and most preferably from 3 to 15 dl/g.
The polymer dispersion according to the invention is stable, and its viscosity does not increase too much during any preparation step. By using the oxidized starch, the carboxylic groups formed in the oxidation can also be utilized in the stabilization of the polymer. Because of the higher molecular weight of starch, it stabilizes the particle also sterically. Furthermore, the ionic groups formed in the oxidation and cationization stabilize the particle, and thus the water-soluble monomers are not necessary for the stabilization of the particle in the polymerization. In the enzymatic treatment of the starch, such stabilizing groups are not formed and the steric stabilization is disappeared as well, and then it is necessary to use emulsifiers, which are generally known to impair sizing. At the same time, the strength properties of the paper are drastically impaired.
During the preparation of the polymer dispersions according to the invention, the starch is preferably cationized by a suitable cationizing chemical depending on the intended use so that in surface sizing applications the DS values are in the range from 0.01 to 1.0, preferably from 0.01 to 0.08, and in pulp sizing applications in the range from 0.05 to 1.0, preferably from 0.08 to 1.0, and more preferably from 0.1 to 0.5. However, a native or anionized starch can be used as well.
If the starch is cationized, suitable cationizing chemicals include cationizing chemicals containing quaternary nitrogen, for example 1,3-epoxy or 1,3-hydrochloride derivatives. 2,3-Epoxypropyltrimethyl ammonium chloride is a preferable cationizing chemical. Cationizing is performed when the starch is in soluble form in an alkaline aqueous solution (pH greater than 7, preferably from 9 to 11). Then the aqueous starch solution has a solids content of under 50%, preferably from 10 to 35%, and a temperature of over 60xc2x0 C., preferably from 75 to 90xc2x0 C. The oxidation and cationization of the natural starch can also be performed simultaneously, but it is preferred that the oxidation is performed before the cationization, because the cationization preferably occurs at higher temperatures than at which the oxidation has to be performed at first because of the gelatinization of native starch.
We have discovered that the best results in co-polymerization are achieved when, unlike in the prior known processes, a starch having a higher molecular weight (i.e. a higher intrinsic viscosity) is used together with a suitable degree of substitution achieved by cationic and/or anionic substituents, and a suitable monomer composition. In the present invention, a suitable monomer composition provides the copolymer to be formed with a good film forming capacity at the minimum film forming temperature (MFT) ranging fromxe2x88x9250 to 200xc2x0 C., preferably from 0 to 100xc2x0 C., more preferably from 0 to 70xc2x0 C., even more preferably from 10 to 50xc2x0 C., and most preferably from 20 to 50xc2x0 C. Most preferably, a mixture is used containing at least two or more polymerizable monomers, of which at least one monomer is a vinyl monomer. It should be noted that the degree of cationization or an ionization of the starch affects the film forming temperature of the polymer, for example, the higher the cationicity of the starch, the lower the film forming temperature of the polymer dispersion. The starch is preferably added at the beginning of the polymerization, but a portion of it can be added later during a step known as the propagation step of the polymerization.
The monomers used can be, for example, styrene, alpha-methylstyrene, acrylates, acrylonitrile, vinyl acetate etc. A monomer mixture is preferable in which at least one of the two monomers is a hydrophobic monomer, such as styrene.
Therefore, the invention relates to a polymer dispersion which consists of the following components, based on the solids content of the product:
a) from 5 to 50%, preferably from 5 to 40% of starch with a degree of substitution (DS) relative to the cationic or anionic substituents from 0.01 to 1 and an intrinsic viscosity, when cationized and/or anionized, of  greater than 1.0 dl/g,
b) from 50 to 95%, preferably from 60 to 95%, of a monomer mixture containing at least one vinyl monomer, the film forming temperature of the polymer, which comprises these components, being from xe2x88x9250 to 200xc2x0 C., preferably from 0 to 100xc2x0 C., and more preferably from 0 to 70xc2x0 C., and
c) water.
The substituents by which the degree of substitution mentioned above is achieved are preferably cationic, and the starch can further contain additional anionic substituents. The degree of substitution defined above can correspondingly be achieved by anionic substituents, and the starch can further contain cationic substituents.
The degree of substitution, particularly the degree of cationicity, of the starch in the polymer dispersion described above is from 0.04 to 1.0, and the intrinsic viscosity is from 1.5 to 1 5 dl/g. A preferable monomer mixture consists of from 40 to 70% of acrylates and from 30 to 60% of styrene.
The invention also relates to a polymer dispersion which consists of from 5 to 50%, preferably from 5 to 40% of starch, from 0 to 19% of acrylonitrile, from 10 to 60% of acrylates and from 10 to 60% of styrene, based on the solids content of the product, and water.
A preferable polymer dispersion according to the invention consists of from 15 to 40%, preferably from 15 to 35%, of starch, from 5 to 19% of acrylonitrile, from 20 to 50% of acrylates and from 20 to 40% of styrene, based on the solids content of the product, and water.
A particularly preferable polymer dispersion according to the invention contains, based on the solids content of the product, 20% of a starch with a degree of substitution of about 0.05 and an intrinsic viscosity of from 3 to 15 dl/g, 19% of acrylonitrile, 30% of acrylates, preferably butyl acrylate and/or 2-ethylhexylacrylate, 31% of styrene, and water.
An aqueous polymer dispersion, which is ready for use, can have, for example, a solids content of from 10 to 60%, preferably from 20 to 50%, more preferably from 25 to 40%, and most preferably from 25 to 35%.
In the process according to the invention, in order to prepare a polymer dispersion mentioned above, a monomer mixture comprising at least one vinyl monomer is copolymerized in an aqueous solution of a starch, and the polymer thus formed has a film forming temperature of from xe2x88x920 to 200xc2x0 C., preferably from 0 to 70xc2x0 C. The polymerization of the monomers onto the starch can also be performed by the sequential steps of adding each monomer type separately to the reaction mixture and allowing it to be polymerized onto the starch before the addition of the next monomer.
One advantage of the invention is that during the polymerization the viscosity is not increased too much, even if the molecular weight is high. In addition, it has been contemplated that when the degree of substitution of the ionic groups onto the starch is slightly higher and the molecular weight of the starch is high, it has been possible to reduce the amount of acrylonitrile in particular, even below 19%. In this case, it has been discovered that the polymerization evidently proceeds by a mechanism known as the precipitation polymerization. In this mechanism, hydrophobic groups are bound onto the starch molecules, which are completely dissolved in water, and these groups try to detach from the water phase. This causes the precipitation of the chains into initial particles. This stage is called the first polymerization step. Then the initiation occurs in an aqueous solution.
The initiators used can be initiators known per se, for example ammonium or potassium persulfate or peroxides, preferably copper sulfate-hydrogen peroxide redox pair. The polymerization temperature is from 70 to 90xc2x0 C., preferably from 75 to 80xc2x0 C., and the pH is below 7, preferably from pH 3 to 5. The addition of the starch is from 2 to 200%, preferably from 10 to 100%, more preferably from 10 to 60%, and most preferably from 10 to 50%, based on the amount of the dry monomer mixture.
The second polymerization step starts when the polymer embryos formed grow and the amount of monomer which is diffused into the growing initial particles is increased. This step is known as the propagation step of the polymerization. The coagulation of the particles is continued until the ionic groups start to stabilize the formed particles. This stabilization in accordance with the DLVO theory results in a stabilized amount of particles and a particle size which is set at an energetically stable range in a way known in colloidal chemistry. The initial particles thus formed act as initial particles for the growing polymer particles. In this case, the mechanism is functioning even without any surface-active agents. In the patent DE 3,702,712, surface-active agents are required as well, which proves that in that case the polymerization proceeds according to the classical Harkins"" model (W. D. Harkins, J. Am. Chem. Soc., 69 (1947) 1428), in which the monomers are first situated in micelles, wherein the initiation starts.
The third polymerization step starts when the free monomer has been lined and it is only present in particles. This is known as the termination step. By now the amount, diameter, and surface charges of the polymer particles have been formed.
By using the process according to the invention a copolymer with high strength is provided having a novel composition and branches of suitable length of a synthetic polymer which are polymerized at a suitable distance from each other. Surprisingly, also the amount of the formed homopolymer is quite small.
By using this polymerization method, paper with a good adhesion and cohesive strength is achieved. In addition to that, in the paper making process, the polymer has a good retention and adherence to fibres, and it improves the initial wet strength, which property is especially required by new, fast-speed xe2x80x98gap formerxe2x80x99 machines containing e.g. shoe presses. Another clearly observed feature is that polymers made by this procedure make the paper machine cylinders less dirty than most of the other synthetic polymers. In this invention, the surface structure of the polymer is assumed to be such that it enables the formation of hydrogen bonds between the particles and the fibre, but it is, however, easier to be removed from the surface of machinery parts by washing.
Both in the paper pulp and surface sizing applications the polymer has been found to be so fully compatible with the pulp or surface sizing starch used that while the polymer is drying, no separation occurs. Then the paper strengths are at their best.
In surface sizing applications, about from 1 to 10% of polymer is generally used, based on the amount of surface sizing starch. Then the mixture of starch and polymer is normally added to the paper surface by using a size press. Especially in fine paper applications, a more even printability is achieved when the polymer is totally admixed with the surface sizing starch. The ink-jet properties of paper are improved as well, i.e. the paper has a better suitability for colour ink-jet printing. The adherence of polymer to paper fibre can be further improved by slightly increasing the degree of cationicity of starch used in polymer stabilization.
In the following examples, starch 1 is a peroxide-oxidized potato starch having an intrinsic viscosity of 9 dl/g, starch 2 is a hypochlorite-oxidized potato starch having an intrinsic viscosity of 1,5 dl/g, and starch 3 is a hypochlorite-oxidized potato starch having an intrinsic viscosity of 9 dl/g. The solids content of all starches is 82%.